I've seen games where you put your mouse over a gui and then a bunch of guis slide from the side into your screen. How do you do that?
There is a really simple function for that which is in the roblox API itself. It's name is
TweenPosition. The only argument you need there is the Position where the GUI should "tween" (interpolate) to. You can also add further parameters to edit the style of this animation.
The wiki article has some really clear information on it: http://wiki.roblox.com/index.php?title=TweenPosition_(Method)